1817 (adj.), 1835 (n.), from French, from Spanish vasco (adj.), from vascon (n.), from Latin Vascones (Vasconia was the Roman name for the up-country of the western Pyrenees), said by von Humboldt to originally mean "foresters" but more likely a Latinized version of the people's name for themselves, euskara or eskuara.

This contains a basic element -sk- which is believed to relate to maritime people or sailors, and which is also found in the name of the Etruscans .... [Room, "Placenames of the World," 2006]

Earlier in English was Basquish (1610s, noun and adjective); Baskles (plural noun, late 14c.); Baskon (mid-15c.).